It is known to use relatively long mineral fibers in thermal insulating materials in the construction industry. Primarily basaltic fibers are used for thermal insulating purposes or for reinforcing of concrete products. Such basaltic relatively long fibers are also known to be used for making support plates or substrates for electronic components.
European Patent Publication EP 0,181,996 A2, U.S. Pat. No. 4,615,733, and Russian Patent Publication RU 2,182,605 C1 disclose the use of fiber-reinforced composite materials with a metal matrix. The fibers embedded in the matrix are short and distributed at random. Thus, the orientation of the short fibers relative to each other is also random. The conventionally used short fibers are generally made of a mineral material with substantial proportions of silicon oxide (SiO2), aluminum oxide (Al2O3), and iron oxide (Fe2O3). However, conventional fiber-reinforced composite materials with short fibers in a metal matrix do not have the mechanical characteristics required, for example in aircraft construction. Such mechanical characteristics include, for example a substantial tolerance relative to damages, particularly a toughness against crack formations and a resistance against fatigue effects, such as fatigue crack propagation.
In the construction of lightweight structural components emphasis is always on the weight reduction, particularly in the aircraft industry. Moreover, and depending on the respective field of application, such composite materials must meet different requirements with regard to their static and dynamic fatigue characteristics including their tolerance to damages. This requirement applies, particularly in the aircraft construction where lightweight structural components must tolerate damages to avoid failure of the aircraft. An improvement of these damage tolerance characteristics can be achieved in different ways, for example by increasing the skin thickness of an aircraft body or body component. The use of additional locally distributed stiffening components helps increasing the damage tolerance. Adapting the skin thickness in those local positions where stress is largest helps improving the damage tolerance. However, all these measures do not necessarily satisfy weight limitations. Hence, there is a need for a compromise, which particularly in the aircraft industry, is not readily acceptable. Another possibility of increasing the damage tolerance characteristics of such composite materials is the use of materials having inherently better damage tolerance characteristics. Materials having such characteristics are metallic laminates or fiber-reinforced laminates.
Recently, fiber-reinforced composite materials with a metal matrix are achieving an increasing significance because in such materials the fibers permit increasing the strength of metallic materials. More specifically, the damage tolerance characteristics of metallic materials can be significantly increased by reinforcing fibers. However, such an improvement is achieved with noticeably higher costs for such metal based fiber composite materials. One important reason for the higher costs lies in the higher production costs. Particularly, production methods in which the metal matrix is melted onto the fibers, involve a substantial effort and expense with regard to production times and production costs. Such costs have been reduced in a relatively economical production method in which sheet metal layers are bonded to each other by an intermediate adhesive film containing the reinforcing fibers.
In this connection reference is made to European Patent Publication EP 0,312,151 disclosing a laminate comprising at least two sheet metal layers with a synthetic adhesive layer between the sheet metal layers, whereby the adhesive layer bonds the sheet metal layers to each other. The adhesive bonding layer comprises glass filaments. Such laminates are particularly useful for lightweight construction in the aircraft industry because these laminates have advantageous mechanical characteristics while simultaneously having a low structural weight.
European Patent Publication EP 0,056,288 discloses a metal laminate in which polymer fibers are used in the bonding layer. These fibers are selected from the group of aramides, polyaromatic hydrazins, and aromatic polyesters in a synthetic material layer.
European Patent Publication EP 0,573,507 discloses a laminated material in which reinforcing fibers are embedded in a synthetic material matrix. The reinforcing fibers used in EP 0,573,507 are selected from a group of carbon fibers, polyaromatic amide fibers, aluminum oxide fibers, silicon carbide fibers, or mixtures of these components.
The above described sheet metal laminates, if compared with equivalent monolithic sheet metals have the advantages of noticeably higher damage tolerance characteristics. For example, metal laminates reinforced with long fiber bonding layers have crack propagation characteristics that are smaller by a factor of 10 to 20 as compared to respective crack propagation characteristics of monolithic sheet metals. On the other hand, these known laminated materials have frequently static characteristics that are worse than those of monolithic materials. For example, the elastic fatigue limits relative to a tension load or pressure load or a shearing load, are lower by about 5 to 20% compared to respective characteristics of equivalent monolithic materials. The fatigue limits of these known laminated materials depend on the use of the type of the bonding or adhesive system and on the types of fibers used in the system.
Efforts to improve the static characteristics of conventional fiber-composite materials are burdened by higher costs. Conventional manufacturing methods, such as powder metallurgical methods or embedding of fibers in a melted matrix material are very cost sensitive. Moreover, the size of conventional fiber-reinforced composite materials producible by the just-mentioned two methods, are rather limited.